1. Field of the Invention
The present invention is directed to a process to remove nitrates from a water source. In particular, the present invention is directed to a process of passing nitrate contaminated water through a strong acid cation exchange resin and thereafter passing the effluent through a weak base anion exchange resin in order to reduce nitrate content while reducing hardness and reducing dissolved solids in the resulting water.
2. Prior Art
Many water sources contain excessively high levels of nitrate, hardness and salt, which makes the water unsuitable for domestic, industrial and agricultural use. Runoff and wells in agricultural areas are especially high in nitrate, hardness and salt. It is desirable to reduce all three, while producing secondary product streams.
Salt is one of the most expensive chemicals to remove from water. In addition, the resulting concentrated salt brine is difficult and expensive to dispose of Producing commercial grade sodium chloride or returning the salt to the ocean is quite expensive. A major source of salt pollution is water softeners. Therefore it is desirable to produce soft water without the use of chemicals that contain chloride or sodium ions.
Two common methods of removing nitrate are: ion exchange and biological denitrification.
Denitrification destroys the nitrate fertilizer while failing to reduce the hardness or salinity of the water. In addition, it introduces a host of organic materials into the water, which adds to water treatment cost. Biological denitrification requires monitoring to insure bacterial health. The biological denitrification requires large expensive equipment. It also requires a carbon source such as methanol or sugar, to support the bioremediation process.
Ion exchange uses a strong or weak base ion exchange resin in chloride form. It exchanges chloride for nitrate and sulfate, which increases the chloride concentration of the treated water. Regeneration requires a large excess of sodium chloride. Therefore the regeneration brine waste contains excessively high amounts of sodium and chloride; which makes it unacceptable as a fertilizer while costly and difficult to dispose of.
Typical regeneration methods yield several bed volumes of salt brine, which is too dilute for practical transport. Therefore, the salt brine must be concentrated using various methods such as distillation or reverse osmosis. Complete regeneration of weak base ion exchange usually requires a large excess of ammonia and several bed volumes of rinse water.
Nitrates are usually removed from water using a strong base anion ion exchange resin which is regenerated with a large excess of salt. The resulting brine is a dilute mixture of mostly sodium chloride and a minor amount sodium nitrate The low fertilizer concentration and high salt content make it unacceptable as fertilizer. This waste brine must be transported disposed in deep salt water injection wells at great expense. Plus the waste brine presents a long term environmental risk.
Strong acid resin and weak base resin have been used to remove nitrate, but have failed to produce a byproduct that has commercial value. Others have added acid to the water before passing it through a weak base resin to improve nitrate removal. This comes at the cost of reduced nitrate removal capacity if sulfuric acid is used. If hydrochloric acid is used, the resulting fertilizer and product water is higher in chloride. However, this increases the total dissolved solids (TDS) of the water and actually compromises nitrate removal by adding competing ions such as chlorides. Some have removed hardness from the water to improve nitrate removal. However, the water softener produces waste brine that is high in salt and adds sodium to the water.
Nitrate are removed with reverse osmosis and once again there is the issue of the waste which is even higher in nitrate and salt. Reverse osmosis waste is usually greater than traditional ion exchange waste. It is too dilute and too high in non nitrate salts to have any fertilizer value. So it must be transported and disposed of as hazardous waste.
Nitrates are also removed from high nitrate water sources through biological means which produces nitrogen gas that has no economic value. Biological methods require long residence times, which translates into very large water treatment facilities. Plus, they produce a wide variety of organic environmental pollutants. Many denitrification plants also require an expensive carbon source such as methanol, glycerin or sugar to work properly.
Notwithstanding the foregoing, there remains a need for a system and process to remove nitrates including passing nitrate contaminated water through a strong acid cation exchange resin followed by passing through a weak base anion exchange and including regeneration procedures.
Further, there remains a need for a system and process for nitrate reduction which includes regeneration of the ion exchange resins to produce high value by products.